Hammer drills are power tools that can generally operate in three modes of operation. Hammer drills have a tool bit that can be operated in a hammer mode, a rotary mode and a combined hammering and rotary mode. For the hammer and combined hammer and rotary mode, it is necessary to convert the rotary motion of the output shaft of the tool motor into a reciprocating motion of a piston, as the piston is used to create an air spring effect to act on a ram which converts the reciprocating motion of the piston into a hammering action.
A mechanism for converting the rotary motion of the output shaft of a motor into a hammering action is described in GB1343206. Referring to FIG. 21 which shows a cross sectional view of the drive mechanism of GB1343206, FIG. 22 which shows a partial cross sectional view of the drive mechanism of FIG. 21 and FIG. 23 which shows a cross sectional view taken along line V-V of FIG. 22, an electric hammer 101 has a motor housing 102 with a driving motor and a gear unit (not shown). A hollow cylindrical guide sleeve 107 has a tool holder 108 that slidably holds a piston-like impact body 110 and a cylindrical shaft 111, which receives impacts from a cup-shaped striker 113.
A piston 114 is slidably disposed inside the cup shaped striker 113, which is slidably mounted in the guide sleeve 107. The piston 114 comprises a rod 120, which is driven by the motor to cause the piston head 116 to reciprocate inside the cup shaped striker 113. This causes an air spring effect to occur forwardly of the piston head 116 so that the striker 113 is caused to reciprocate under the air spring effect. The reciprocation of the striker 113 is transmitted to the impact body shaft 111 and the impact body 110 to cause a hammer action that is transmitted to a tool bit (not shown).
Referring to FIGS. 22 and 23, the outer surface of the striker 113 comprises a plurality of flat surfaces 128 and a plurality of part-cylindrical surfaces 129. The part-cylindrical surfaces 129 slidably engage the internal cylindrical surface of the guide sleeve 107. The flat surfaces 128 do not engage the internal cylindrical surface of the guide sleeve 107 and effectively reduce the area of contact between the striker 113 and the guide sleeve 107. This reduces friction between the striker 113 and guide sleeve 107 to increase the efficiency of the drive mechanism.
The drive mechanism of GB1343206 suffers from the drawback that the gaps between the flat surfaces 128 and the internal cylindrical surface of the guide sleeve 107 are relatively large, and can play no part in the function of the drive mechanism other than reduce surface area contact and perhaps assist air-flow inside the drive mechanism of GB1343206.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.